Voltage followers are known from prior art, they also being termed impedance converters. Voltage followers are DC amplifiers employing negative feedback, designed so that the output voltage Ua follows the input voltage Ue. In making use of voltage followers the source furnishing the input voltage Ue is exclusively loaded by a very high input impedance of the voltage follower whilst the output voltage Ua of the amplifier originates from a source having a low output impedance, it being from this source that currents can then be obtained. The voltage follower does not alter the level of the input voltage, it instead facilitating further processing thereof by reducing the source impedance. Shown in FIG. 1a is a voltage follower known from prior art as described e.g. on page 327 of the German textbook “Elektronik” published by Heiner Herberg, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig/Wiesbaden, 2002.
The operational amplifier used in this prior art voltage follower may be e.g. an operational amplifier formed by MOSFETs as shown in FIG. 1b. In this arrangement the input voltage Ue is applied to the gate of a first NMOSFET MN1. The negative feedback is achieved by the drain of a second NMOSFET MN2 being connected to its gate which also forms the output of the voltage follower at which the output voltage Ua is available and which follows the input voltage Ue. The two NMOSFETs MN1 and MN2 are connected to each other by their sources, the connecting point of which is connected to ground via the source drain circuit of a third NMOSFET MN3. This third NMOSFET MN3 serves as a current source and is signaled by a bias voltage applied to its gate. In addition, 2 PMOSFETs MP1 and MP2 are provided in a current mirror configuration serving as the active loads of the operational amplifier and the source-drain circuit of which is connected to a supply voltage potential Vcc.
One disadvantage of the voltage follower as shown in FIG. 1b is that the output voltage Ua is no longer able to follow the input voltage Ue once the output voltage has assumed very low values, when, for instance, being below the threshold voltage Vt of a NMOSFET since the NMOSFET MN2 is then no longer conducting. This is particularly a disadvantage in circuits working with low supply voltages.